In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air interface to the user equipments in downlink (DL) transmissions.
In LTE the radio base station decides on the transport parameters or format including modulation and code rate to use for all data transmissions but may also be assisted in its choice of transport parameters by terminal or user equipment reports. One of the reports available is the Channel Quality Indicator (CQI) report. In the CQI report the UE indicates a suggested modulation and coding to use given the current radio condition, considering both signal and interferences strength. Making accurate CQI estimates in a UE is difficult and costly, hence the system must compensate for errors in the CQI. Errors in CQI can also reside from offset in interference between symbols used for channel estimation and symbols used for data transmissions, referred to as interference mismatch. An additional error comes from the time delay between CQI estimation before the report is available in the radio base station.
In order to compensate for the errors in CQI an outer-loop may be applied. The outer-loop takes in additional feedback from Automatic Repeat Request (ARQ) protocols like the Hybrid ARQ (HARQ) or the Radio Link Control Acknowledged. Mode (RLC AM) in LTE, and uses that additional feedback to calculate a correction value also called back-off value to the estimated channel quality based on deviation from expected error rate.
With packet oriented systems, such as LTE, one issue with the channel estimation for data transmission is the fast varying interference situation. The interference variations due to varying load in neighboring cells can be handled in two different ways; if no communication to the other cell is possible the transport format selection must take the additional variations into account, by applying a larger back-off value or by other means. If fast communication between cells is possible interference prediction can be applied. A common way to compensate for the interference error is to base the correction value on UE estimated path-loss measurements to the neighboring cell. This can in situations with large interference variations show significant throughput gains compared to uncoordinated systems. Large gains can be achieved by coordinating the interfering cells in a LTE system. Updated channel quality estimates may be made by converting CQI estimates to Signal to Interference plus Noise ratio (SINR) estimates and then update the channel quality values according to the new interference situation given that the path loss to the own and neighboring cells are known. Path loss to neighboring cells can in LTE be calculated from Received Signal Reference Power (RSRP) measurements designed for handover control. An RSRP report may comprise a filtered estimate of the received power from neighboring cells.
A problem with compensating CQI using RSRP is that RSRP reports also contain errors and it also takes time for the UE to make the required estimates so it may take hundreds of ms before the RSRP reports are available in the radio base station. Thus, the performance of the radio communication network is reduced due to the delay and that the RSRP report comprises errors.